In general, an electric motor drive device for driving an electric motor by an inverter has a smooching capacitor provided in parallel with a DC power supply on the inverter input side. The smoothing capacitor serves to reduce the impedance of a main circuit and suppress surge voltage, and also serves to absorb regenerative power in the case of abnormality and prevent overvoltage in the main circuit. One example of abnormal cases is a so-called load dump in which, while the electric motor is performing regenerative operation, input wiring of the inverter is disconnected due to vibration or the like, or a breaker provided on the input side fails, and thus connection between a smoothing capacitor on the inverter input side and a DC power supply is opened.
If the capacitance of the smoothing capacitor is reduced for the purpose of size reduction, increase in voltage caused by absorption of regenerative power in the case of abnormality becomes great, leading to overvoltage.
As a conventional technique for suppressing such overvoltage, an overvoltage protection device is provided which includes: a bypass resistance circuit interposed between terminals at an input part of an inverter; a relay interposed in series to the bypass resistance circuit and controlled, to be opened/closed; and control means for controlling the relay to make the bypass resistance circuit active when input voltage of the inverter is equal to or greater than reference voltage (see, for example, Patent Document 1).
A conventional electric motor drive device using another technique includes: a regenerative power processing circuit connected in parallel to a smoothing capacitor between a positive side and a negative side of a DC power supply; a braking power processing circuit connected to an AC power supply line extending from an inverter bridge to an electric motor; and a common resistor which also serves as a braking resistor included in the regenerative power processing circuit and used for processing regenerative power from the electric motor (see, for example, Patent Document 2).
In a conventional electric motor drive device using still another technique, when abnormality has occurred in which a DC circuit leading to an inverter from a battery as a power supply is opened, for example, switching elements of an upper arm or a lower arm of the inverter are collectively turned on to output a zero-voltage vector, thereby short-circuiting input terminals of an electric motor and stopping supply of power from the inverter (see, for example, Patent Document 3).